Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize its surface. The charged portion of the photoconductive surface is exposed to a light image from a scanning laser beam or a LED diode that corresponds to an original document or image being reproduced. The effect of the light on the charged surface produces an electrostatic latent image on the photoconductive surface.
After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises a mixture of magnetic carrier granules and toner particles that adhere triboelectrically to the latent image. A single-component developer material is typically comprised of toner particles without carrier particles. Toner particles are attracted to the latent image, forming a toner powder image on the latent image of the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet to form the hard copy image.
In color electrophotographic printers, one or more Raster Output-Scanner (ROS) units are positioned adjacent to a moving photoreceptor belt surface or the like and are selectively energized to generate successive image exposures. Each of the latent images is representative of one of a plurality of different color separation images. Development of the color separation latent images and the transfer of the developed image to an output media define a multi-color image. The color separation images may define, for example, yellow, magenta, cyan, and black color images that, upon subtractive combination on the output media, produce a visible representation of the multi-color image. When an observer looks at the output media, the human eye integrates the light reflections from the color component images into shades of color.
For a color imaging cycle, the printer generates a component image exposure for each color component of the desired output image. For full color images, four component images may be generated, one component image for each of the three basic colors and a fourth for black images. Each component image is transferred to the moving photoconductive surface by modulating the light source of an ROS as it moves across the moving photoconductor to selectively discharge the photoconductor in an image-wise pattern. The modulation control of the laser or LED of the ROS is provided by the video data. For each color component image, the value of an image data byte is used to enable or disable the laser or LED that illuminates the photoreceptor for one pixel timing period. If the data value is less than a threshold value, the LED or laser is not operated for one pixel period. If the data value indicates that a pixel is to be generated, then the LED or laser is operated for one pixel period.
An ROS that generates a series of pixels comprising a color component image places the color component image it generates in overlying registration with the other color component images. These overlapping images are composed of a series of closely spaced pixels that are nominally spaced 5 microns apart. When these component images are developed at development stations to transfer toner particles of different colors to the component images, a composite color image is formed. This composite color image is then transferred to an output sheet. Thus, the quality of color image perceived by the observer relies upon the precise registration of the component image pixels.
In some electrophotographic imaging systems, the latent images of the component image are formed and developed on top of one another in a common imaging region of the photoconductor. The latent images generated in the common imaging region may be formed and developed during multiple passes of the photoconductor around a continuous transport path. Alternatively, the latent images can be formed and developed in a single pass of the photoconductor around the continuous transport path. A single-pass system enables multi-color images to be assembled at extremely high speeds. In either type of imaging systems, accurate placement of the color separation image pixels in the latent image is important for image quality.
One problem with the pixels generated by these types of ROS devices is the lack of precision regarding the size and placement of the pixels. Because the entire data byte is mapped to only one value, either an on or off value, only one of two types of pixels may be produced. Therefore, the data bytes for an image may define pixels of different densities, but the ROS is only able to generate a pixel that is either on for the full duration of a pixel clock cycle or off for the same period of time. Consequently, some of the data content for an image is lost in the process of generating a latent image from a set of image data.
In order to enhance the quality of an image in high speed color laser printers, it is necessary to control the size of the pixels. This is done by changing the timing and the duration of the laser beam for each particular pixel. The technique to provide such a function is called “Sub-Pixel Generation”.
Published Application No. 2006/0215188 A1, Sep. 28, 2006, herein incorporated by reference, discloses a method and system for sub-pixel generation in a high speed laser electrophotographic system. The output of the sub-pixel generating circuit disclosed therein as applied to the ROS is officially adjusted for each pixel in order to control pixel size. This system can be advantageously implemented in a full custom mixed-mode (analog/digital) integrated chip (SPG chip). However, such a full-custom chip is optimized for one particular desired printing speed and also comprises for the higher speeds of present electrophotographic laser systems, a rather delicate and expensive custom-made chip circuit.
As alternative printing system speeds may be desired, there is a need for advantageously still utilizing the foregoing existing SPG custom chip, but for generating different speeds, which would avoid having to develop a new full custom specialized integrated chip for each desired speed.
Accordingly, it is desired to utilize an existing SPG chip for different or higher speeds with changing only a few off-chip components that may be associated with the chip so that a system can be utilized which quickly and inexpensively switches between the different printing speeds.